A little more information might be helpful!

Firstly, do you have the interrupt vector pointing to an interrupt service routine? Otherwise the interrupt will fire off into no-mans land and the 8051 may just execute nops until the program counter gets back to some real code. Or, if you have thevector set correctly and an interrupt service routine, are you ressting the RI and TI bits? Otherwise the routine will keep on firing and thus slowing things down appreciably. Or, you may be trying to do too much work in the interrupt service routine thus using a lot of cpu time.

Some example code might allow us to be more specific....